In lithographic printing, a printing plate is mounted on the cylinder of a printing press. The printing plate carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the printing plate onto a receiver material, typically a sheet of paper. Generally, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the receiver material (offset printing).
In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. When the surface of the printing plate is moistened with water and ink is applied, the hydrophilic regions retain water and repel ink, and the ink-receptive regions accept ink and repel water. During printing, the ink is transferred to the surface of the receiver material upon which the image is to be reproduced.
Lithographic printing plates typically comprise an imageable layer (also called imaging layer or imaging coating) applied over the hydrophilic surface of a substrate, typically aluminium. The imageable layer includes one or more radiation-sensitive components, often dispersed in a suitable binder.
To produce the lithographic image on the printing plate, the printing plate is imaged by targeted radiation. This can be carried out in different ways. In direct digital imaging (computer-to-plate), printing plates can be imaged with infrared or UV lasers or light sources. Such a laser or light source can be digitally controlled via a computer; i.e. the laser can be turned on or off so that imagewise exposure of the precursor can be affected via stored digitized information in the computer. Therefore, the imageable layers of printing plates, which are to be imagewise exposed by means of such image-setters, need to be sensitive to radiation in the near-infrared region or UV of the spectrum.
The imaging device will thus etch the image on the printing plate by eliciting a localized transformation of the imageable layer. Indeed, in such systems, the imageable layer typically contains a dye or pigment that absorbs the incident radiation and the absorbed energy initiates the reaction producing the image. Exposure to the imaging radiation triggers a physical or chemical process in the imageable layer so that the imaged areas become different from the non-imaged areas and development will produce an image on the printing plate. The change in the imageable layer can be a change of hydrophilicity/oleophilicity, solubility, hardness, etc.
Following exposure, either the exposed regions or the unexposed regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. Developers are typically aqueous alkaline solutions, which may also contain organic solvents.
Alternatively, “on-press developable” lithographic printing plate can be directly mounted on a press after imaging, and are developed through contact with ink and/or fountain solution during initial press operation. In other words, either the exposed regions or the unexposed regions of the imageable layer are removed by the ink and/or fountain solution, not by a developer. More specifically, a so-called on-press development system is one in which an exposed printing plate is fixed on the plate cylinder of a printing press, and a fountain solution and ink are fed thereto while revolving the cylinder to remove the undesired areas. This technique allows an imaged, but un-developed printing plate (also called a printing plate precursor) to be mounted as is on a press and be made into a developed printing plate on an ordinary printing line.
If the exposed regions are removed, the precursor is positive-working. Conversely, if the unexposed regions are removed, the precursor is negative-working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and do not accept ink.
On-press developable negative-working lithographic (offset) printing plates are known in the prior art.
For example, U.S. Pat. No. 5,569,573 teaches lithographic printing plates comprising a laser imaging layer containing microencapsulated oleophilic materials in hydrophilic polymer binders.
EP 0 770 495 teaches lithographic printing plates comprising near infrared absorption materials, polymer binders and thermoplastic particles capable of coalescing under heat.
U.S. Pat. No. 6,983,694 teaches on-press developable negative-working offset printing plates coated with near infrared sensitive coating compositions comprising thermoplastic polymer particles, such as polystyrene or poly(acrylonitrile-co-styrene) particles, non-reactive hydrophilic polymer binder and near infrared absorption dyes.
U.S. Pat. No. 6,261,740 teaches a non-process negative working laser imageable lithographic offset printing plate having radiation-sensitive composition coated on a hydrophilic substrate. The radiation-sensitive composition comprises copolymers having acid catalyzed pendant groups, which were polymerized from N-alkoxy methyl methacrylamide, and 3,4-epoxycyclohexyl methyl methacrylate. It further comprises phenolic binder resins, iodonium salt as acid generator, near infrared absorbing dye, visible dyes and film forming additives. Upon exposure to near infrared laser light, a crosslinking reaction occurs via cationic polymerization. The unexposed area could be developed on press with fountain solution.
Also, U.S. Pat. Nos. 6,124,425 and 6,177,182 teach on-press developable negative-working offset printing plates coated with thermally near-infrared absorbing polymers, which undergo crosslinking reactions via cationic polymerization upon exposure to near infrared radiation. The near infrared chromophoric moieties are functionalized to the polymeric backbone via ether and ammonium bonds.
U.S. Pat. No. 6,960,422 teaches negative-working offset printing plates containing a near infrared sensitive base-coat composition comprising molecular near infrared dyes, radical generators, radical polymerizable urethane compounds, reactive polymer binders and other additives.
Moreover, U.S. Pat. Nos. 6,969,575 and 7,001,704 teach on-press developable negative-working offset printing plates having an image-forming layer that comprises near infrared absorbing microcapsules and an acid generating compound.
U.S. Pat. Nos. 6,582,882, 6,846,614, and 6,899,994 and U.S. Patent application 2005/0123853 teach on-press developable negative-working offset printing plates coated with thermally imageable compositions containing polymer binders, initiator systems and polymerizable components. The described polymer binders are copolymers having non-reactive polyethylene oxide and polypropylene block, or graft copolymers having non-reactive polyethylene oxide side chains co-polymerized with acrylonitrile, styrene and other monomers. The polymerizable components are viscous liquid oligomers containing multiple acrylic functional groups. The initiator system contains near infrared absorption dyes and radical producing compounds, such as triazine and iodonium salts.
U.S. Pat. No. 7,261,998 teaches on-press or off-press developable negative-working offset printing plates comprising an image-forming layer, which comprises near infrared absorbing dyes having a tetraaryl pentadiene chromophore, a polymeric binder comprising a hydrophobic backbone to which poly(alkylene glycol) side chains are directly or indirectly linked, and free radical generating iodonium salt. The image-forming layer further comprises, as an adhesion promoter, a nonionic liquid phosphate acrylate having a molecular weight of at least 250.
U.S. Patent Application No. 2009/0186299 teaches a negative-working imaging coating composition that comprises an initiator element, a near infrared radiation absorbing compound, a polymeric binder and an adhesion promoter to increase the printing durability of the coating composition. The described adhesion promoters are liquid organic compounds having an ethylenically unsaturated carbon-carbon double bond that is connected to an alkoxysilyl or hydroxysilyl group, such as vinyltrimethoxysilane, vinylmethyldimethoxy-silane, vinyltriethoxysilane, vinyltris(2-methoxyethoxy)silane, vinyltriacetyloxy-silane, 3-acryloxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxy-silane, and 3-methacryloxypropylmethyldimethoxysilane.
U.S. Patent Application No. 2009/0111051 teaches a negative-working imaging coating composition including an initiator element, a near infrared radiation absorbing compound, a polymeric binder and a stabilizing composition. The stabilizing composition comprises liquid poly(ethylene glycol) diacid and free radical reactive compounds containing ureido terminated group, such as Sipomer WAM II from Rhodia (USA) and 1-[N-[poly(3-alkoxy-2-hydroxypropy)]-2-aminoethyl]-2-imidazolidinone from Aldrich Chemical Company (USA).
Positive working lithographic (offset) printing plates containing near infrared laser radiation sensitive polymeric coatings are also known in the prior art. See for example, WO 97/39894, EP 0 823 327, EP 0 909 657, WO 98/42507. These documents taught to prepare heat sensitive coatings comprising Novolak and (meth)acrylate type polymeric substances, near infrared absorbing compounds and dissolution inhibiting compounds. The near infrared absorbing and dissolution inhibiting compounds prevent the polymeric substance from dissolving in the liquid developer. This is due to the formation of a network structure via hydrogen bonding and/or ionic interactions within the coating composition. Upon imaging with near infrared laser light, this network structure within the exposed areas is disrupted and dissolves faster in the liquid developer compared to the non-exposed areas (image areas).
However, the solubility difference between the exposed and non-exposed areas may sometimes vary during storage and usage of the plate. Different approaches have been taught in the prior art toward overcoming the above problems.
For example, U.S. Pat. No. 6,461,795 teaches to treat the lithographic printing plates before shipping to the customers at a preferred temperature between 50 and 60° C. in low relative humidity for several hours to accelerate the formation of a stable network structure within the coating composition. This heat treatment process however increases the lithographic printing plates production cost and time.
U.S. Pat. No. 6,613,494 teaches to apply a thin over-layer to protect the non-exposed areas of the polymeric coating from attack by the liquid developer. Again, this approach increases the production cost and time of the lithographic printing plates.
U.S. Pat. No. 6,420,087 teaches to prepare coating compositions containing siloxane compounds as image protecting agent in order to reduce the dissolution of the non exposed areas during developing. The presence of these siloxane compounds caused however some phase separation in the coating solution making it difficult to coat this composition on substrates, for example with the roller coating techniques and pinhole. In addition, such siloxane compounds are not soluble in the alkaline developers. This causes sludge buildup in the processor and redeposit on the printing plates and shortens the lifetime of developer.
WO 2004/020484 teaches to prepare coating compositions consisting of acetal copolymers containing carboxylic acid, sulfonic acid and phosphoric acid terminated pendant groups, Novolak resin, near infrared absorption dyes, visible dye and image protecting agent for production of high chemical resistant thermally sensitive positive working lithographic offset printing plates. Such coating compositions require post heat treatment at 50° C. for one day in order to keep the image area from being attacked by the developer.
U.S. Pat. Nos. 6,255,033 and 6,541,181 teach to prepare and use acetal copolymers containing carboxylic acid, hydroxy, halide, methoxy, and acetylene functional groups as binder resins for production of positive-working lithographic offset printing plates that can be imaged with near infrared laser radiation. Such coating compositions require an adhesive promotion agent and a near infrared absorbing dye as dissolution inhibitor. In practice, high loading levels of near infrared dye and visible dye are used to differentiate between exposed and non-exposed areas during development. Moreover, the presence of such small organic molecules in the coating compositions might cause phase separation during coating. It also reduces the mechanical strength and causes blooming during storage.
U.S. Pat. Nos. 6,124,425 and 6,177,182 teach to prepare heat sensitive polymeric coating compositions for positive-working lithographic printing plates comprising near infrared absorbing chromophores grafted on the backbone of Novolak, acrylate and methacrylate based polymers. Optionally, the coating compositions may contain other binder resins and film-forming additives. The use of near infrared absorbing polymers in the thermally sensitive polymeric coating compositions exhibited several advantages, such as quick formation of stable network structure, good resistance of the non-exposed area to attack by the liquid developer without the need for a heat treatment or a protective over-layer.
U.S. Pat. No. 7,473,515 teaches to prepare heat sensitive polymeric coating compositions for positive-working lithographic printing plates comprising near infrared absorbing chromophores grafted on the backbone of acetal co-polymers. Optionally, the coating compositions may contain Novolak binder resins, colorants and film-forming additives.
U.S. Pat. No. 7,544,462 teaches to prepare heat sensitive polymeric coating compositions for positive-working lithographic printing plates comprising phenolic or acetal polymer binder resins, near infrared absorbing dyes and low molecular development enhancing compounds, such as dialkylamino benzoic acid.
U.S. 2009/0004599 teaches to prepare heat sensitive polymeric coating compositions for positive-working lithographic printing plates comprising acetal polymer having cyclic-ester pendant groups to improve resistance to press chemicals, such as alcohol substituted fountain solutions, UV wash solutions and UV inks.
WO 99/11458 also teaches about positive-working lithographic offset printing plates.
Despite all these advances in the art, there remains a need for new materials and new coatings for lithographic printing plates.